1. Field of the Invention:
The invention in general relates to sonar hydrophones, and in particular to a large conformable hydrophone array for use with beam forming apparatus.
2. Description of the Prior Art:
A need exists for a sonar system to precisely detect distant underwater targets without the requirement for an active transmission of an acoustic pulse. The need is met by a passive array of hydrophones in conjunction with beam forming apparatus to pinpoint target location based upon the self-noise generated by the target.
Generally, the larger the array aperture the greater will be its ability to accurately determine target location. If the array is carried by an underwater vessel such as a submarine, the array should be conformable to the submarine shape so as not to interfere with its hydrodynamic design.
Conformable arrays have been built utilizing individual piston-type hydrophones having a piezo ceramic active element. For extremely large arrays, however, such construction is prohibitively heavy, and the active elements are subject to breakage in the presence of a shock wave. In addition, such array is reflective of incident acoustic energy thereby making it easily detectable.
To obviate these shortcomings, there has been proposed an array made up of relatively flat flexible piezoelectric elements of a piezoelectric polymer such as polyvinylidene flouride (PVF.sub.2). The PVF.sub.2 elements forming the array are lightweight, shockproof and flexible so as to conform to a curved base structure. The use of such flexible elements, however, has produced less than satisfactory results over the frequency range desired for detecting distant targets. The response and beam patterns formed utilizing the PVF.sub.2 elements have not been in conformance with theoretical expectations and this behavior is unacceptable for controlled beamformer operation.
The hydrophone array of the present invention utilizes lightweight, flexible piezoelectric elements in a structure which minimizes inter-element coupling, has uniform and high element sensitivity, controlled element beam patterns, has little element-to-element variation and is acoustically transparent.